Author: Fraser Cain

Although Galileo wasn’t able to see the phases of Mercury (like the phases of Venus and the Moon) with his first crude telescope, the astronomers that carried on his discoveries did. This was powerful evidence that both Mercury and Earth are orbiting the Sun.

You can also check out these amazing books for more information about the planet Mercury.

Since Mercury orbits the Sun, and it follows a very elliptical path on its journey around the Sun, our two planets can vary their distance significantly.

When Mercury is at its closest point to Earth, astronomers call this opposition (from the point of view of Mercury). This would happen when Mercury was at its furthest from the Sun, and Earth is at its closest. When this happens, Mercury and Earth would be separated by only 77 million km (48 million miles).

Their maximum distance occur when Earth is at its furthest point from the Sun, and Mercury is at its maximum on the other side of the Sun. The three objects then line up perfectly. At this point, Mercury and Earth can be 222 million km (138 million miles) apart.

Distancia desde la Tierra a Mercurio

Venus is one of the 5 planets visible to the unaided eye, and so it has been seen in the sky since prehistoric times. After the Sun and the Moon, Venus is the brightest object in the sky – brighter than any stars.

Venus is the Roman name for the Greek goddess of love; Aphrodite. But there are references to Venus as far back as Babylonian cuneiformic texts, like the Venus table of Ammisaduqa which could be as old as 1600 BC. Ancient Egyptians thought that Venus was actually two separate objects… a morning star, and then a completely different evening star. The tradition carried on with the ancient Greeks, as well.

The early mathematician Pythagoras was one of the first to recognize that the morning and evening stars were actually the same object: Venus. So perhaps he was the first to “discover Venus”.

The Maya civilization held Venus in high regard, and the planet figured prominently in their religious calendar.

Once he built his first rudimentary telescope, Galileo turned it towards Venus, was was astonished to find that the planet goes through phases like the Moon, going from crescent to gibbous to full and then back again. This was one of the strongest pieces of evidence that Venus goes around the Sun, and not the Earth as others originally believed.

Because Venus has always been obscured by thick clouds, science fiction writers have been free to imagine whatever they wanted existed on the surface of Venus. The clouds helped them to imagine a warm tropical world with constant rainfall and lush vegetation. The truth, of course, is that Venus is a hellish hot world, devoid of life.

We’re familiar with the close-up images of Earth captured by orbital satellites and astronauts on the International Space Station. But here are a few pictures of the Earth and Moon captured at a distance, while passing around the Moon, or orbiting distant Saturn. In the words of Carl Sagan, “Look again at that dot. That’s here. That’s home. That’s us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives.”

Since Sagan commented on Voyager 1’s image of Earth, several more pics of the Earth and Moon have been captured that really put our tiny spot in the Universe into perspective.

Voyager 1 – the Pale Blue Dot
This image, captured by NASA’s Voyager 1 spacecraft on February 14, 1990, is the one Sagan was talking about. It was taken when Voyager 1 was about 6.4 billion kilometres (4 billion miles) from Earth. In this image, the entirety of the planet takes up less than a single pixel; in fact, NASA calculated that we’re only 0.12 pixel in size. Sagan pushed the agency to capture images of all the planets in the Solar System, to create a family portrait seen from Voyager’s distant position.

Cassini – the Earth from Saturn
This is easily the best picture ever captured by NASA’s Cassini spacecraft, and in the running for the best space pictures of all time. It shows Saturn in all its glory, backlit by the Sun. From this view, the rings glow with an ethereal light.

But this image is extra special. Take a look towards the upper left of the image, and you’ll notice a tiny blue-white dot. Once again, there’s Earth, seen from a distance of 1.4 billion km (1 billion miles).

Mars Express – the Earth and Moon from Mars
ESA’s Mars Express captured this image of Earth and the Moon on July 3, 2005 when it was 8 million km (5 million miles). This was actually the first observational data sent back by the probe. A fitting introduction to Martian system. Of course, Mars Express was designed to image the relatively nearby surface of Mars, and not such distant objects, so the resolution of the image isn’t very good. And yet, they’re instantly recognizable as the Earth and Moon.

Opportunity Rover – the Earth from the Surface of Mars
Here’s an image of Earth captured from the surface of another planet. In this case, we’re seeing Earth from Mars, as photographed by NASA’s Mars rover Opportunity on April 29, 2005. Normally operating only in the Martian day, Opportunity woke up a little after sunset, and captured this image of Earth as the stars were starting to come out. The image of Earth looks a little elongated because it was moving slightly during the 15-second exposures. Imagine what some future Martian might see, stepping out onto the surface of the Red Planet in the dusky twilight to see our bright planet in the sky.

Rosetta – Earth and Moon during a flyby

Now this is what you might have been expecting to see. A picture of Earth, with the continents clearly visible. This photograph of Earth and the other of the Moon were taken by ESA’s Rosetta spacecraft during a flyby on November 15, 2007. Rosetta’s primary mission is to visit Comet 67P/Churyumov-Gerasimenko in May 2014, but this flyby was a great opportunity for Rosetta to test out its OSIRIS camera system. The image of the Moon was taken separately.

Earthrise – seen from the Moon
This is the classic “Earthrise” photograph, captured by astronaut William Anders during the historic Apollo 8 mission – the first human mission to the Moon. This photograph was taken on December 24, 1968. If you’re standing on the surface of the Moon, the Earth doesn’t actually “rise”. Since the Moon is tidally locked to the Earth, our planet always hangs in exactly the same point of the sky. It takes an orbiting spacecraft to see the Earth actually rise from the horizon.

This photograph was recreated by the Japanese Kaguya spacecraft, which captured a similar sequence of the Earth using its high-definition video camera.

Earth from Galileo
And finally, I think I’ve saved one of the best pictures for last. This is a photograph of the Earth and Moon (not to scale) captured by NASA’s Galileo spacecraft as it was speeding away towards Jupiter. Galileo captured these images in 1992, and finally reached the Jovian system in 1995-97. You’re looking at the Pacific Image, with South America visible as well as the Caribbean.

For most of human history, Mercury was a mystery, wrapped in superstition. As it orbits so close to the Sun, ancient astronomers could only see it when they had an open horizon, just after the Sun sets, or before it rises.

But now we’ve sent robotic spacecraft to all of the planets in the Solar System, including Mercury. Here’s a listing of the missions that have gone to Mercury, and a few that will be going shortly.

Mariner 10
Mariner 10 was launched on November 3, 1973 to fly past Mercury and Venus. It was the last spacecraft in the Mariner program (Mariner 11 and Mariner 12 were renamed to Voyager 1 and 2).

Its main objectives were to measure Mercury and Venus’ environment, atmosphere and capture images of their surfaces (the cloud tops of Venus).

It was the first spacecraft to take advantage of the gravity assisted slingshot maneuver, using the gravity of Venus to bend its flight path to bring it into a good trajectory to fly past Mercury. It also used light pressure from the Sun to make minor course corrections.

It made a flyby of Venus on February 5, 1974, and then its first Mercury flyby on March 29, 1974, getting within 703 kilometers of the planet’s surface. It completed a second flyby on September 21, 1974, and then a third and final flyby on March 16, 1975, passing as close as 327 km. Over the course of the three flybys, it was only able to map 40-45% of Mercury’s surface.

Mariner 10 is probably still orbiting the Sun.

MESSENGER
MESSENGER is NASA’s second mission to Mercury. It launched on August 3, 2004 to study the planet. This time, the spacecraft will actually be going into orbit around Mercury to study it until its electronics fail.

The spacecraft has greatly improved optics and electrons, and is capable of resolving features on Mercury’s surface down to 18 meters across. This is a vast improvement over the 1.6 km resolution of Mariner 10.

MESSENGER made its first flyby of Mercury on January 14, 2008, and will make a second on October 6, 2008, and a third on September 29, 2009. It will make its final orbital insertion around Mercury on March 18, 2011.

Bepi Columbo
This is a new mission under development by the European Space Agency and the Japan Aerospace Exploration Agency to the planet Mercury. It’s still in the planning stages, but ESA and JAXA intend to build a spacecraft that can split into two portions: a planetary orbiter and a magnetospheric orbiter. A planetary lander component was proposed, but the idea was scrapped for budgetary reasons.

Misiones a Mercurio

References:
NASA Solar System Exploration: Mariner 10
NASA Messenger Mission Page
NASA: Bepi-Colombo